The lack of atomic structural information has become one of the major roadblocks in understanding ATP synthesis. This project is attempting to overcome this block through the elucidation -- by single crystal x-ray diffraction methods.-- of the structure of the F1-sector of the ATP synthase, the portion of the enzyme that contains all the catalytic and nucleotide binding sites. At the present level of resolution (3.6 Angstroms) density has been found for about 900 of the 1025 residues of the major subunits. This information has produced the most detailed description available of the quaternary structure of the F1-sector including the identification of alpha and beta subunits. Recent success in obtaining improved crystals has opened the door to extend the resolution of this study to 3.3 Angstroms. An electron density map at this higher resolution will resolve many of the present ambiguities in the chain connectivity; it will then be possible to complete tracing of the polypeptide chain and to refine the structure by crystallographic methods. The catalytic and nucleotide binding sites of the enzyme will be identified by inspection of the final structure and by studying complexes of the F1 with non-hydrolyzable ATP analogs using difference Fourier methods. This information will be used for evaluating existing mechanisms for ATP hydrolysis and synthesis and for proposing new mechanisms for experimental evaluation.